This invention relates to a nozzle vane of a radial inflow turbine, and more particularly to a reverse curved nozzle vane.
Typically, the nozzle vane of a radial inflow turbine for an Air Cycle Machine (ACM) has an airfoil shape formed by a concave inner surface, also referred to as the pressure surface; and either a convex or flat outer surface, also referred to as the suction surface. These nozzle vanes usually require brazing or welding between the nozzle and the housing assembly. Sometimes, rather than brazing or welding, mounting bolts may be used, which may reduce assembly cost, and may provide for increased or more secure containment. Containment is a special structure requirement for most of the aircraft applications, i.e., when a wheel breaks due to overspeed in any adverse circumstances, all the broken parts with high kinetic energy need to be contained in a housing structure. However, a nozzle vane head (also called the leading edge) that uses mounting bolts is usually larger than a nozzle vane head using brazing or welding. And, a larger head may lead to excessive aerodynamic losses.
A main function of the turbine nozzle is to convert the flow static head to dynamic head by accelerating the flow in both the radial direction and the tangential direction. Static head is an expression for the energy of the fluid per unit mass due to the fluid static pressure difference between two reference points. The dynamic head, also referred to as the “velocity head”, is proportional to the square of the velocity of the working fluid. The sum of the above two heads usually refers to the total head, provided both reference points, the inlet and exit of a pipe, or a nozzle, or a turbine, are at the same elevation. For an ideal flow through a nozzle, the total head would be constant, even though the static head and the dynamic head would change over the length of the nozzle. But in reality, total head decreases along the flow path inside the nozzle due to wall friction, boundary layer growth, and boundary layer separation.
Aerodynamic losses incurred in the above conversion process are also due to incidence when flow enters nozzle vane passage at an angle. For a well-designed airfoil-shaped nozzle vane with a thin leading edge, the main aerodynamic loss may be from vane surface friction. However, when the large rounded leading edge required by mounting bolts is present with a traditional wedge-type nozzle vane (as seen in prior art FIG. 1) or a traditional inward-concave/outward-non-concave vane profile (as seen in prior art FIGS. 2 and 3), the loss from the boundary layer separation may be predominant even at the design condition due to immediate flow over-turn after the large leading edge.
U.S. Pat. No. 5,299,909 discloses a radial turbine nozzle vane that has a pressure surface (inner surface) having a concave inwardly portion after the nozzle throat, and the convex outwardly shape at its suction surface (outer surface), as seen in FIG. 2 of the '909 patent.
U.S. Pat. No. 6,491,493 discloses an axial flow turbine, as opposed to a radial inflow turbine. The '493 patent discloses a preferred width of nozzle blades, and the spacing between nozzle blades, as seen in the '493 patent FIGS. 1 and 2.
U.S. Pat. No. 6,887,041 discloses axial flow turbines for gas turbine engines, and all the nozzle vanes have a conventional airfoil shape. The '041 patent discloses a particular airfoil-shaped vane profile by x-y-z coordinates as seen in Table 1.
As discussed above, each of these patents would have the disadvantages of excessive aerodynamic loss due to incidence and boundary layer separation if an large leading edge is introduced to accommodate the mounting hole for low-cost assembly and enhanced containment reasons.
FIG. 1 illustrates a prior art wedged nozzle vane 10. This prior art wedged nozzle vane 10 has a straight outer surface 11 and a straight inner surface 12. This type of design can be made at a lower cost but has excessive aerodynamic loss due to the unfavorable flow acceleration schedule inside the nozzle flow passage.
FIG. 2 illustrates a prior art semi-airfoil nozzle vane 20. This prior art semi-airfoil nozzle vane 20 has a straight outer surface 21 and a concave inner surface 22. This type of design can accommodate mounting bolts at its large leading edge but will result in excessive aerodynamic loss due to the immediate flow over-turn right after the leading edge.
FIG. 3 illustrates an airfoil nozzle vane 30. This prior art airfoil nozzle vane 30 has a convex outer surface 31, and a concave inner surface 32. This type of design can have reduced aerodynamic loss, but usually does not permit the use of mounting holes due to the vane thickness and radial constraint.
As can be seen, there is a need for a nozzle vane that does not lead to excessive aerodynamic losses, that has improved containment features, and has reduced assembly cost.